1. Field of the Invention
The present invention relates to a servo control method and a servo control system for a feed drive system, and particularly, to a servo control method and a servo control system for a feed drive system of a machine tool for a high speed, high precision processing.
2. Description of Relevant Art
The machine tool includes a feed drive system and a servo control system for the feed drive system. The servo control system has three control loops for feedback control, i.e. a position loop as a major loop and speed and current loops as minor loops.
For feedback control of position, there are known a semi-closed loop system, a full-closed loop system and a hybrid control system. The semi-closed loop system employs a rotation angle signal of a servo motor, as a position feedback signal. The full-closed loop system employs a signal of a position scale for detecting an actual position of the machine, as a position feedback signal. The hybrid control system is a hybrid of the semi-closed loop system and the full-closed loop system.
The full-closed loop system carries all errors of the feed drive system (a mechanical system) in the control loop, allowing for a high precision to be expected. However, as the control loop contains many non-linear factors, the system tends to be unstable and has a sacrificed responsiveness for a stable action.
To this point, the semi-closed loop system does not directly take in complicated mechanical system performances, and is allowed to have an increased response speed with a secured stableness. However, as all errors of the feed drive system (a mechanical system) are not carried in the control loop, those errors left outside the loop are not covered for compensation. Like condition appears in the hybrid control system, which also has an error involved in a feeding movement.
Machine tools for a high speed, high precision processing employ servo control systems for their feed drive systems. Typically, most of those servo control systems are constituted as a semi-closed loop system or a hybrid control system, which however are disabled to compensate for an involved error in a feeding movement of a mechanical system, and subjected to a limitation on a potential desire for an enhanced processing route accuracy (configuration accuracy).
Such the limitation tends to be greater in significancy, as a feed speed is modified higher, with an increasing desire for a servo control system of a semi-closed loop system or a hybrid control system to be capable of a compensation for an involved error of a mechanical system, along with an increasing demand for a higher speed in mechanical processing, as well as for a higher precision.
Mechanical systems tend to have an involved error greater in significancy, as an employed feed speed is higher.